5
2.3 Global Positioning System GPS
2.3.1 GPS Background Information
The GPS System was created and realized by the American Department of Defense DOD and was originally based on and run with 24 satellites 21 satellites being required
and 3 satellites as replacement. Nowadays, about 30 active satellites orbit the earth at a distance of 20200 km Hoffman-Wellenhof, B, H. Lichtenegger and J. Collins 2001.
GPS satellites transmit signals which enables a GPS receiver to extract its location, if it is positioned on the surface of the earth, in the earth atmosphere or in a low orbit. GPS
is being used in aviation, nautical navigation and for determing the orientation of shore. The GPS signal can be used without a fee by any person in possession of a GPS receiver.
Furthermore, it is used in land surveying and other applications where the determination of the exact position is required. At first only 18 satellites were in operation. In 1988 the
number of satellites is raised to 24, as the functionality is not satisfactory with only 18 satellites Thuong Le-Tien, Vietnam in 2010.
The first Block I satellite carried sensors to detect atomic explosions. This satellite is meant to control the abidance of the agreement of 1963 between the USA and the Soviet
Union to refrain from any nuclear tests on the earth, submarine or in space. When a civilian airplane of the Korean Airline Flight 007 was shot down after it had gone lost over Soviet
territory, it was decided to allow the civilian use of the GPS system. In 1986, the accident of the space shuttle Challenger meant a drawback for the GPS program, as the space
shuttles were supposed to transport Block II GPS satellites to their orbit. Finally the operators of the program reverted to the Delta rockets which were intended for the satellite
deployment in the first place.
In 1989, the first Block II satellite was installed and activated. Temporal deactivation of the selective availability SA was done during the Gulf war. In this period
civilian receivers were used since not enough military receivers were available. On July 01, 1991 it was activated again. The Initial Operational Capability IOC was announced
6 in 1993 Thor Hogan, Vic Villhard 2004, p. 12. In the same year it was decided to
authorize the world wide free of charge usage for civilian.
The last Block II satellite completed the satellite constellation in 1994. Full Operational Capability FOC was announced the following year. In the year 2000, final
deactivation of the selective availability was done and therefore improving the accuracy for civilian users from about 100 m to 20 m resolution.
2.3.2 Longitude and Latitude Co-Ordinate System
Kategori